Add Support For Additional Search Filters

[apavlo]

There are the additional meta-data fields for systems that we are not able to search on right now. The only meta-fields that we currently support are the following:

1. Compatible With: https://dbdb.io/browse?compatible=mysql
2. Derived From: https://dbdb.io/browse?derived=tikv
3. Inspired By: https://dbdb.io/browse?inspired=spanner
4. Country: https://dbdb.io/browse?country=US

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We need to add filtering for these additional fields:

1. Operating System
2. Programming Languages
3. Project Type
4. Licenses

We also need to support mixing the filters. It looks like we support that now but the search result label only includes the one of the filters:

https://dbdb.io/browse?derived=postgresql&country=US

This because the template code is just a bunch of if-else statements:

https://github.com/cmu-db/dbdb.io/blob/master/templates/core/database-browse.html#L113-L149

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I don't know if there is a more generic way we should be performing these searches or whether it should be ad-hoc like we are doing now:

https://github.com/cmu-db/dbdb.io/blob/master/dbdb/core/views.py#L307-L346

[pembo13]

@apavlo I think I can come up with a generator for the English language representation of the search. Adding the new filters should be simple enough, but everything together should be 1hr to 2hrs.

I'll work on this after #45

[pembo13]

The index will need to be rebuilt: python manage.py rebuild_index --noinput