Faceting fixes

[etj]

• Some fixes for faceting (see commit messages)
• Implementation of topics as per Facet topics #11105

Commits should be preserved (i.e. not squashed) since their scope is quite specific.

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 #11191 (744d8ba) into master (e103d06) will increase coverage by 0.05%.
The diff coverage is 92.18%.

❗ Current head 744d8ba differs from pull request most recent head 94d25ea. Consider uploading reports for the commit 94d25ea to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #11191      +/-   ##
==========================================
+ Coverage   61.18%   61.23%   +0.05%     
==========================================
  Files         839      839              
  Lines       51798    51850      +52     
  Branches     6648     6653       +5     
==========================================
+ Hits        31691    31752      +61     
+ Misses      18581    18565      -16     
- Partials     1526     1533       +7