Limit push email diff size

Restrict the size of diffs in push emails to 30 lines / 150 KB (whichever is smaller),
following https://gitlab.com/gitlab-org/gitlab_git/merge_requests/85.

Emails on push can get very large if they contain a lot of files that sneak under the single-file limit.

https://gitlab.com/gitlab-org/gitlab-ee/issues/490

See merge request !4566

Fix RangeError exceptions when referring to issues or merge requests outside of max database values

When using #XYZ in Markdown text, if XYZ exceeds the maximum value of a signed 32-bit integer, we get an exception when the Markdown render attempts to run `where(iids: XYZ)`. Introduce a method that will throw out out-of-bounds values.
    
Closes #18777

See merge request !4777

When using #XYZ in Markdown text, if XYZ exceeds the maximum value of a signed 32-bit integer, we
get an exception when the Markdown render attempts to run `where(iids: XYZ)`. Introduce a method
that will throw out out-of-bounds values.

Closes #18777

1. An exception would be raised if the filter was called with an invalid
   URI. Mainly because we weren't catching the `Addressable` exception.

2. This commit fixes it and adds a spec for the filter.

Avoid that GitHub import fails when retrieving Webhooks for non GitHub admins

Closes #18729 

See merge request !4723

Docs also added.