Show 503 page on blogs under high load

Fix Novel pagination

Clean up ActivityPub mentions

Private instance cleanup

Accessibility improvements for readers

OAuth account management

- Break up linked / to-link sections
- Add logos for all services
- Lay out buttons horizontally
- Tweak the copy

Ref T713

This includes the chosen GitLab display name in the button text.

This alters the V6 migration to make the column NULLable. Anyone who has already run this migration will need to manually update their database.

Clean up GitLab auth code

This acknowledges "too many connections" and "max user connections"
errors in MySQL and propagates the error up the chain so we can notify
the user and return the correct HTTP code.

This passes in the correct field named GitlabDisplayName.

This makes the app less error-prone by avoiding a regexp.MustCompile()
call in the ActivityObject() method, saves CPU work, and reuses code.

This fixes log formatting and makes verbiage consistent & concise.

This ensures handles are always stored without leading @ symbol.

Previously, we might potentially return a nil activitystreams.Object,
which would crash the app. This fixes that.

This darkens the text slightly to get a contrast ratio over 4.5:1.

This also darkens the text color of the blog description, to
differentiate it from the pinned links.

Add 'X-Robots-Tag: noindex' header to invite URLs

lightly style tables in posts

Add Gitlab OAuth

Don't show date on pinned post page

Fix V4 + V5 SQLite migrations

Ref T669

Typography Improvements, Vol. I

We'd like these queries to fail correctly if the tables exist.

This index needed a unique name in order for this query to succeed.

Previously, this migration would cause the error: "Cannot add a NOT NULL
column with default value NULL". This fixes that by setting the default
value for new columns to '' (empty string). It updates the query builder
to support this, too.

Combining all operations into a single query was causing problems in
SQLite. This fixes that by breaking them up into separate queries. It
also moves one column length change to only run on MySQL, since SQLite
doesn't need it.

It's not straightforward to remove these constraints in SQLite, so this
just skips it entirely. Since both of these migrations are part of the
same WF release, this should have minimal impact on admins.

This previously used a default timestamp value which caused the
migration to fail for SQLite databases.

Adjust Reader limits and filtering

This moves logic for determining whether or not to show a "Sign up" page
on /signup (i.e. because the `/` route shows another, non-signup page)
into the AppCfg.SignupPath() method. It also changes various signup
links to use this value.

(that is, hide the footer nav link when logged out)

Refactor CLI