[Fixes #12124] GNIP 100: Assets [2]

[etj]

First implementation of assets.

Also these changes are needed: GeoNode/geonode-importer#235

Replaces https://github.com/GeoNode/geonode/pull/12179 since this PR branches from a more recent master commit.

Checklist

Reviewing is a process done by project maintainers, mostly on a volunteer basis. We try to keep the overhead as small as possible and appreciate if you help us to do so by completing the following items. Feel free to ask in a comment if you have troubles with any of them.

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 #] 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[bot]]

Codecov Report

Attention: Patch coverage is 80.61798% with 138 lines in your changes are missing coverage. Please review.

Project coverage is 64.31%. Comparing base (33ea4b9) to head (baf134f). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #12260 +/- ## ========================================== + Coverage 64.13% 64.31% +0.17% ========================================== Files 872 884 +12 Lines 52319 52904 +585 Branches 6480 6529 +49 ========================================== + Hits 33553 34023 +470 - Misses 17275 17379 +104 - Partials 1491 1502 +11 ```

[mattiagiupponi]

closed in favor of https://github.com/GeoNode/geonode/pull/12338