https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5620#note_13671791

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5620#note_13540099

Closes #20115

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

* This method previously iterated over all services in a project.  Now it
will directly query the ExternalWikiService for the project and filter
by active state.
* The presence of an external wiki is also cached
* When an external wiki is added or removed, the cached value is updated

Requested in #14669

Added note to changelog

This reverts commit 13e37a3e.

This ensures that code such as this don't run needless SQL queries:

    project.gitlab_issue_tracker_service.project

This also means that if the root `project` eager loads any associations
the Service object will be able to re-use those.

Using update_column to store the boolean flag to avoid
any side effects with the current state of the project
instance

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

In 8278b763 the default behaviour of annotation
has changes, which was causing a lot of noise in diffs. We decided in #17382
that it is better to get rid of the whole annotate gem, and instead let people
look at schema.rb for the columns in a table.

Fixes: #17382

update changelog

This greatly improves the performance of Project#external_issue_tracker
by moving most of the fields queried in Ruby to the database and letting
the database handle all logic. Prior to this change the process of
finding an external issue tracker was along the lines of the following:

1. Load all project services into memory.
2. Reduce the list to only services where "issue_tracker?" returns true
3. Reduce the list from step 2 to service where "default?" returns false
4. Find the first service where "activated?" returns true

This has to two big problems:

1. Loading all services into memory only to reduce the list down to a
   single item later on is a waste of memory (and slow timing wise).
2. Calling Array#select followed by Array#reject followed by Array#find
   allocates extra objects when this really isn't needed.

To work around this the following service fields have been moved to the
database (instead of being hardcoded):

* category
* default

This in turn means we can get the external issue tracker using the
following query:

    SELECT *
    FROM services
    WHERE active IS TRUE
    AND default IS FALSE
    AND category = 'issue_tracker'
    AND project_id = XXX
    LIMIT 1

This coupled with memoizing the result (just as before this commit)
greatly reduces the time it takes for Project#external_issue_tracker to
complete. The exact reduction depends on one's environment, but locally
the execution time is reduced from roughly 230 ms to only 2 ms (= a
reduction of almost 180x).

Fixes gitlab-org/gitlab-ce#10771

Password can now be specified at the same time as the new URL, and the service
template admin pages now work.

This reverts commit b4639754.

It now allows to specify a password at the same time as the new URL, and works
on the service template admin pages.

Issue gitlab-com/support-forum#213

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