[Fixes #7842] Rename layer to dataset

[mattiagiupponi]

References: #7842

This PR is meant to introduce a big change in the code, by renaming layers to datasets.
There is still usage of the word Layer in the code (especially in the variables), but this is the first implementation.
The main changes are:

• Renaming as much as possible of the world Layer. This includes:
  -- Models: from Layer to Dataset
  -- Foreign Key
  -- Properties: from map.layers to map.datasets
  -- Methods: from def layer_metadata to def dataset_metadata
  -- Forms: from LayerForm into DatasetForm
  -- Management commands: from set_all_datasets_public to set_all_datasets_public
  -- Tests refactor
  -- CircleCI build yml
  -- Permissions name

NOTE: the requirements.txt will be updated once the client PR is ready. Otherwise, the build (and the app) will crash
client PR: GeoNode/geonode-mapstore-client#343

The issue will be created soon

Checklist

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[codecov]

Codecov Report

Merging #7841 (89e5d33) into master (3692cc5) will increase coverage by 0.00%.
The diff coverage is 63.71%.

@@           Coverage Diff           @@
##           master    #7841   +/-   ##
=======================================
  Coverage   56.62%   56.63%           
=======================================
  Files         691      698    +7     
  Lines       43565    43612   +47     
  Branches     5583     5585    +2     
=======================================
+ Hits        24670    24700   +30     
- Misses      17458    17473   +15     
- Partials     1437     1439    +2     

[lgtm-com]

This pull request introduces 1 alert when merging 999ba95 into 3692cc5 - view on LGTM.com

new alerts:

• 1 for Unused import

[lgtm-com]

This pull request introduces 1 alert when merging b3dce52 into 3692cc5 - view on LGTM.com

new alerts:

• 1 for Unused import