Option to delay conflict checking of old pull requests until page view (#27779)

`[repository.pull-request] DELAY_CHECK_FOR_INACTIVE_DAYS` is a new
setting to delay the mergeable check for pull requests that have been
inactive for the specified number of days.

This avoids potentially long delays for big repositories with many pull
requests. and reduces system load overall when there are many
repositories or pull requests.

When viewing the PR, checking will start immediately and the PR merge
box will automatically reload when complete. Accessing the PR through
the API will also start checking immediately.

The default value of `7` provides a balance between system load, and
keeping behavior similar to what it was before both for users and API
access. With `0` all conflict checking will be delayed, while `-1`
always checks immediately to restore the previous behavior.

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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

options/locale/locale_en-US.ini

@@ -1880,7 +1880,7 @@ pulls.add_prefix = Add <strong>%s</strong> prefix
 pulls.remove_prefix = Remove <strong>%s</strong> prefix
 pulls.data_broken = This pull request is broken due to missing fork information.
 pulls.files_conflicted = This pull request has changes conflicting with the target branch.
-pulls.is_checking = "Merge conflict checking is in progress. Try again in few moments."
+pulls.is_checking = Checking for merge conflicts ...
 pulls.is_ancestor = "This branch is already included in the target branch. There is nothing to merge."
 pulls.is_empty = "The changes on this branch are already on the target branch. This will be an empty commit."
 pulls.required_status_check_failed = Some required checks were not successful.