Add opensearch support

[Leopardus4]

Hi,

It would be awesome to have opensearch support implemented in Libreddit!

Docs for reference.

This PR adds basic OS support. However, I didn't test it locally, so please check that it works as intended (especially relative links).

What this PR does not add: Search suggestions. They would be also cool, but it's not my technical stack. To add it, generating simple JSON is needed: docs. I guess new template would be needed here, similar to search.html.

[spikecodes]

I love this idea, thank you! Tomorrow, I can make Libreddit resolve /opensearch.xml to the XML file and then this will be complete!

[spikecodes]

Unfortunately, the Url parameter in the OpenSearch specification doesn't support local paths so I'd probably have to create a new command line argument to pass in the URL of the instance which would allow the instance to enable OpenSearch support.

[Leopardus4]

Darn, I hoped it will be simpler. Yeah, passing host URL via command line is a good solution here. It looks like invidious & nitter do it this way. Another option I thought about was to get the Host http header from request in runtime (is this possible in Hyper?) - just sayin'

[Scoder12]

+1 for host header

[seychelles111]

bump