Unify search UI

[aexvir]

Right now our search is a bit lacking, it just allows for some filtering on each page, but often is not comfortable or the results can't be searched with enough granularity.

I personally really like the approach that GitLab, Google Search or GitHub have. A single search bar in the top, that allows searching everywhere and provides easy search result categorization.

This would really benefit from having some search backend, like I proposed in #248 , as doing all the searches with other mechanisms doesn't seem that comfortable.

Steps for this:

• Make indexer framework for indexing our models in ES (service, dependency, check category -> overview filtered & bonus points for opening specific issue), take inspiration from checks. Autodiscovery of index_ functions, and calling them periodically (e.g. 1h)
• Write indexers for our current models (Mdl.objects.all() -> index attrs)
• Remove search from all the templates, and the views related to it
• Make new search package with a single view proxying searches to ES
• Write FE search bar component communicating with the backend view via AJAX++
• Write FE results panel component (take inspiration from Sentry's search maybe?)