The "trending" functionality will expand to include trending links and
posts. But at the moment the "Trending" references in the code are
exclusively to hashtags.
Rename "Trending" to "TrendingTags" or similar everywhere necessary in
order to prepare for this.
This includes a database migration, as the identifier for the "Trending
tags" tab in the account preferences was changed from "Trending" to
"TrendingTags". The migration updates the stored value if necessary.
