Advertise the PAT as an alternative unless GH import is not configured.
Signed-off-by: NRémy Coutable <remy@rymai.me>

This stands as an alternative to using OAuth to access a user's Github
repositories.  This is setup in such a way that it can be used without OAuth
configuration.

From a UI perspective, the how to import modal has been replaced by a full
page, which includes a form for posting a personal access token back to the
Import::GithubController.

If the user has logged in via GitHub, skip the Personal Access Token and go
directly to Github for an access token via OAuth.

Requested in #14669

Added note to changelog

Closes #19252

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Closes #19133

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rely only on IssuesFinder
docs and changelog

Fixes gitlab-org/gitlab-ce#14794 gitlab-com/performance#13

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This removes the need for manually updating the list of queries every
time we make a change.

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- Add new images to match the new UI
- Document the new prioritize feature

Add information about manually creating Todos from issue and merge request sidebars.

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