Add setting to enable/disable HTML emails

Closes #24880

See merge request !7749

This new global setting will allow admins to specify if HTML emails should be sent or not,
this is basically useful when system administrators want to save some disk space by avoiding
emails in HTML format and using only the Plain Text version.

When I proposed using serializable transactions I was hoping we would be
able to refresh data of individual users concurrently. Unfortunately
upon closer inspection it was revealed this was not the case. This could
result in a lot of queries failing due to serialization errors,
overloading the database in the process (given enough workers trying to
update the target table).

To work around this we're now using a Redis lease that is cancelled upon
completion. This ensures we can update the data of different users
concurrently without overloading the database.

The code will try to obtain the lease until it succeeds, waiting at
least 1 second between retries. This is necessary as we may otherwise
end up _not_ updating the data which is not an option.

Add a starting date to milestones

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/23704

See merge request !7484

There may be more tables but these were the tables that were problematic
for GitLab.com due to foreign key constraints (without cascading
deletes).

Allow to block JIRA events for commits and merge requests

implements #5541 

See merge request !7469