Show message if version of Central is out-of-date

[matthew-white]

Frontend fetches /version.txt. We could have Frontend show a message recommending an upgrade if the version in version.txt is out-of-date. The message would only be shown to sitewide administrators. (A sitewide administrator might not be able to complete the upgrade themselves, but they probably know who to contact to do so.)

Now that we're using date-based versioning, we could consider checking the year part of the version. The easiest case is if the year part is <= current year - 2, because then the version of Central is definitely quite out-of-date. If the year part is current year - 1, it's hard to know exactly how old the version of Central is without fetching the current version or a list of versions from an external source. If the year part is current year - 1, then the version of Central could be the latest version, or on the other extreme, it could be almost two years out-of-date.

[matthew-white]

We discussed this issue on Tuesday. One idea that came out of that is that we could save a user preference (getodk/central-backend#764) about whether the user has been shown this message. If the user has been shown the message, then we could wait another 30 days or so to show it to them again.

@sadiqkhoja had the idea that we could also use news.html to inform users that their version is out-of-date. The news iframe is currently only 300px x 40px. However, one advantage of using it is that older versions of Central could see the message:

• As soon as possible, start passing a version query parameter to news.html in the iframe.
• After a few releases, modify news.html so that it shows a message if the version query parameter is missing or quite out-of-date.