webapp: remind user when using repositories that lack consistency checks

When starting up the assistant, it'll remind about the current
repository, if it doesn't have checks. And when a removable drive
is plugged in, it will remind if a repository on it lacks checks.

Since that might be annoying, the reminders can be turned off.

This commit was sponsored by Nedialko Andreev.

Assistant/Threads/SanityChecker.hs

@@ -33,8 +33,10 @@ import Data.Time.Clock.POSIX
  - being nonresponsive.) -}
 sanityCheckerStartupThread :: Maybe Duration -> NamedThread
 sanityCheckerStartupThread startupdelay = namedThreadUnchecked "SanityCheckerStartup" $ do
+	{- Stale git locks can prevent commits from happening, etc. -}
 	void $ repairStaleGitLocks =<< liftAnnex gitRepo
 
+	{- If there's a startup delay, it's done here. -}
 	liftIO $ maybe noop (threadDelaySeconds . Seconds . fromIntegral . durationSeconds) startupdelay
 
 	{- Notify other threads that the startup sanity check is done. -}