[Fixes #11101] Faceting: keywords

[etj]

Implements #11101

Creates a new FacetProvider KeywordFacetProvider

Checklist

For all pull requests:

• Confirm you have read the contribution guidelines
• You have sent a Contribution Licence Agreement (CLA) as necessary (not required for small changes, e.g., fixing typos in the documentation)
• Make sure the first PR targets the master branch, eventual backports will be managed later. This can be ignored if the PR is fixing an issue that only happens in a specific branch, but not in newer ones.

The following are required only for core and extension modules (they are welcomed, but not required, for contrib modules):

• There is a ticket in https://github.com/GeoNode/geonode/issues describing the issue/improvement/feature (a notable exemption is, changes not visible to end-users)
• The issue connected to the PR must have Labels and Milestone assigned
• PR for bug fixes and small new features are presented as a single commit
• Commit message must be in the form "[Fixes #<issue_number>] Title of the Issue"
• New unit tests have been added covering the changes, unless there is an explanation on why the tests are not necessary/implemented
• This PR passes all existing unit tests (test results will be reported by travis-ci after opening this PR)
• This PR passes the QA checks: black geonode && flake8 geonode
• Commits changing the settings, UI, existing user workflows, or adding new functionality, need to include documentation updates
• Commits adding new texts do use gettext and have updated .po / .mo files (without location infos)

Submitting the PR does not require you to check all items, but by the time it gets merged, they should be either satisfied or inapplicable.

[codecov]

Codecov Report

Merging #11231 (48959a9) into master (f839d39) will increase coverage by 0.00%.
The diff coverage is 82.85%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #11231   +/-   ##
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  Coverage   61.29%   61.30%           
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  Files         840      841    +1     
  Lines       51934    51966   +32     
  Branches     6669     6672    +3     
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+ Hits        31834    31858   +24     
- Misses      18565    18573    +8     
  Partials     1535     1535