Update patch_versions.md with explicit --with statement for db gem group

## What does this MR do?
Updates `patch_versions.md`'s instructions on installing gems to explicitly install either the postgres or mysql groups.

## Are there points in the code the reviewer needs to double check?
No, but thank you for your time.

## Why was this MR needed?
I performed the steps listed in `patch_versions.md` and ran the command for updating postgres gems. Bundler told me at the end that it had not installed the postgres gem group, which caused the next step, migrating the DB, to fail for obvious reasons.

Changing the command to `--with postgres` made it install the postgres gem group, and db:migrate then passed.

## What are the relevant issue numbers?
None

## Screenshots (if relevant)
N/A

See merge request !3955

# Conflicts:
#	Gemfile.lock

feature proposal issue template in contributing guide

This adds a feature proposal template to the contribution guidelines for internal and external use.

Fixes #15448 

cc @markpundsack @stanhu @dzaporozhets 

See merge request !3967

Enable LstripRstrip cop

This requires no code changes since it doesn't actually change anything in the codebase, just preventative.

See merge request !3959

Instrument methods used in email diffs

See merge request !4038

Remove unnecessary method call in events view

Since `User#to_param` already returns `User#username`, we don't need to
pass in the user's username. Changing this also helps us obey LoD.

See merge request !3996

Initialize wikis on legacy projects during check

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/4173

Helps https://gitlab.com/gitlab-org/gitlab-ce/issues/15423

See merge request !3931

Revert "Remove the Devise Async gem."

This reverts commit 1cc614f2.

It was causing the ActiveJob integration to fail, so unfortunately we'll have to add the gem again.

Resolves #17250.

See merge request !4027

Improve uniqueness of field names on the signup form

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/15075

See merge request !3826

Make all of the nested constant instrumentation for core app code work
the same way, add mailer instrumentation, and add instrumentation to the
premailer gem.

Fixed username links in the performance guide

See merge request !4036

These would end up being rendered as:

    <a href="...">@yorickpeterse</a>
    <a href="...">@yorickpeterse</a>

[ci skip]

Fix a spec that was failing due to !3483

Spec were skipped in this MR so that tests started to fail in master
instead of in this MR!

/cc @connorshea @rspeicher 

See merge request !4033

Spec were skipped in this MR so that tests started to fail in master
instead of in this MR!
Signed-off-by: NRémy Coutable <remy@rymai.me>

Made it clearer that issue_id means iid, not id.



See merge request !4030

Use sign out path only if not empty

Fixes: https://github.com/gitlabhq/gitlabhq/issues/10066

See merge request !3989

Update rspec-rails from 3.3.3 to 3.4.2.

This allows the removal of the monkey patch from this commit: 47ff1c56

It'll also make it slightly easier to upgrade to 3.5.0 later.

Changelog: https://github.com/rspec/rspec-rails/blob/master/Changelog.md#340--2015-11-11

See merge request !3999

Link to GitLab code review guide in contributing guidelines

I noticed that the contributing guidelines still link to the thoughtbot
code review guidelines even though we already have our own.

[ci skip]

See merge request !4018

This reverts commit 1cc614f2.

It was causing the ActiveJob integration to fail, so unfortunately we'll have to add the gem again.

Improve the Two-factor Authentication sign-in text [ci skip]

Resolves #14543.

See merge request !3483

Resolves #14543.

Add clear instructions on installing the pg_trgm extension

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/17191

See merge request !4025

[ci skip]

Added performance guidelines

Fixes #15254 #14277

See merge request !3303

[ci skip]

Updated CHANGELOG for 8.7.2

See merge request !4021

[ci skip]

[ci skip]

I noticed that the contributing guidelines still link to the thoughtbot
code review guidelines even though we already have our own.

[ci skip]

[ci skip]

Support e-mail notifications for comments on project snippets

While working with project snippets recently, I noticed that notifications would not be sent out for comments on notes. This MR fixes this.

Note: I'm not completely sure why `ProjectSnippets#participants` returns an empty array if you don't include the concern that is already in `Snippets` but didn't dig into it any more.

Closes #2334

See merge request !3987

Closes #2334

Fix error when trying to create a wiki page

Closes #15527, #15569, #15623, #15630, #15637, #15653, #15870, #16558, #16875, #16987, #17016, https://github.com/gitlabhq/gitlabhq/issues/10317.

See merge request !3924

Fix "remember me" sign in option

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/4006

Prior to this change, 'remember me' was effectively always on because
the `_gitlab_session` cookie was always made persistent. With this
change it becomes possible again to have `_gitlab_session`be a session
cookie when 'remember me' is unchecked.

See merge request !4004

Signed-off-by: NRémy Coutable <remy@rymai.me>

Remove duplicate entry in the CHANGELOG

[ci skip]

See merge request !4002