Add User-Agent for image proxy

[This client](https://github.com/indieweb/chat.indieweb.org/blob/d24f9be587dc82a4d22ab830e9f273eb2cc00a12/inc.php#L44) should have a custom user agent to identify it as the image proxy of chat.indieweb.org. This is especially important considering the bot flood we have to fight with as website owners.

Start in inc.php around line 44, where the image proxy client is defined. Inspect how the proxy makes its request and add a custom User-Agent identifying it as the image proxy for chat.indieweb.org. Done when outgoing proxy requests use that identifying header.

Written by the indexing model from the issue text.