This reverts merge request !12195

This adds an ID-less table containing one row per file, per merge request
diff. It has a column for each attribute on Gitlab::Git::Diff that is serialised
currently, with the advantage that we can easily query the attributes of this
new table.

It does not migrate existing data, so we have fallback code when the legacy
st_diffs column is present instead. For a merge request diff to be valid, it
should have at most one of:

* Rows in this new table, with the correct merge_request_diff_id.
* A non-NULL st_diffs column.

It may have neither, if the diff is empty.

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/27070

Deprecate "chat commands" in favor of "slash commands"

We looked for things like:

 - `slash commmand`
 - `slash_command`
 - `slash-command`
 - `SlashCommand`

* Note that protected branches will not be part of the deletion

As per d5d8e76b, any attempts to rename a container registry repository are blocked due to image signatures.

This is the first commit doing mainly 3 things:
1. create a new scope and allow users to use it
2. Have the JWTController respond correctly on this
3. Updates documentation to suggest usage of PATs

There is one gotcha, there will be no support for impersonation tokens, as this
seems not needed.

Fixes gitlab-org/gitlab-ce#19219

* implemented logging of project and system web hooks
* implemented UI for user area (project hooks)
* implemented UI for admin area (system hooks)
* implemented retry of logged webhook
* NOT imeplemented log remover

Clarify how the milestone percentage is calculated in the docs.
Include example image of milestone.

This reverts commit b0498c17