Allow custom info to be added to Help page

We can already add text to the Sign in page, but this will be seen by most users very infrequently. If they're looking for help with GitLab, they are more likely to end up at the Help page, where it would be nice to have the ability to add custom info like how to contact the local GitLab admin.

@DouweM I would like to take a jab at this. Or are you already working on this?

@jeroenvanbaarsen No plans to work on it, just filing it away here for whenever :) Feel free to look into it!

Reassigned to @jeroenvanbaarsen

mentioned in merge request !1309 (merged)

Status changed to closed by commit fe17a236

mentioned in commit fe17a236

mentioned in commit 1eb3dde4

mentioned in merge request !14785