improvements and polish for the help filter

[ccbrown]

Expands on #140:

• Overfetches from the database to avoid situations where we're fetching one result at a time, looking for a hit. This reduces load times for nohelp pages from ~5s to ~400ms.
• Simplifies page hashes, e.g. reducing nohelp=true to just nohelp.
• Only shows the help forum toggle of the locale has a help forum.
• Adds localization for the help forum toggle.
• Adds caching with a short TTL to the activity endpoint.