Synapse: Use a specific user-agent for fetching link previews

[lindner]

Synapse fetches with Link Previews with something the default user agent:

Synapse/0.18.7 (b=matrix-org-hotfixes,3c9acde,dirty)

Instead a separate user agent with 'bot' in the name and a link to a page would indicate that these fetches are legitimate. Something like this:

Synapse-LinkFetchBot/0.18.7 (+http://matrix.org/bot.html)

[richvdh]

as noted by #4376, the very specific user agent is also something of a data leak. Including a version string is inappropriate.

[Mikaela]

I guess this issue is why all YouTube Music previews tell:

Your browser is deprecated. Please upgrade.
Sorry, YouTube Music is not optimized for your browser. Check for updates or try Google Chrome. Get Chrome

(Thanks for original English, Davo)