"Broken" forum caching

[Pepo]

Approx. at the same time as the changeset [14735] was introduced and later applied to some projects, those forum web pages appear to be incorrectly/excessively cached. A lot of input can be found on msg boards threads BOINC dev: Cached project forum pages? and ​SETI@Home: Yellow pages still here!. Just briefly:

• An already multiple times displayed thread page must be manually refreshed, in order to be displayed as already read.
• Upon returning to the lists of threads, they do not reflect recently read or updated threads (with new messages). Manual page refresh is necesary to obtain a correct version from project server.
• If a recently visited and read thread is entered again (e.g. using a link from a notification email), it appears to be read (so the user can scratch his/her head and go somewhere else) and a manual refresh is necessary to display newly added messages.

The feeling is: it gets annoying. From the experience, the threshold interval (while the pages appear to be cached) seems to be many minutes.

[davea]

(In [15089]) - user web: send no-cache headers except if we're doing

whole-page caching. Fixes #626