fix: Move autocomplete view for links to admin

[fsbraun]

This PR removes the need for registering its own autocomplete view by moving the autocomplete view to the Link plugin itself.

Note: The link plugin handles the autocomplete view also for other plugins using the LinkPluginMixin.

Expected to fix #209

[codecov]

Codecov Report

Attention: Patch coverage is 80.76923% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 86.39%. Comparing base (f11feee) to head (d1392f3).
Report is 18 commits behind head on master.

Files	Patch %	Lines
djangocms_frontend/contrib/grid/forms.py	0.00%	2 Missing ⚠️
djangocms_frontend/contrib/link/forms.py	81.81%	2 Missing ⚠️
djangocms_frontend/templatetags/frontend.py	0.00%	2 Missing ⚠️
djangocms_frontend/common/title.py	0.00%	1 Missing ⚠️
djangocms_frontend/contrib/carousel/models.py	0.00%	0 Missing and 1 partial ⚠️
...ms_frontend/contrib/icon/templatetags/icon_tags.py	0.00%	1 Missing ⚠️
djangocms_frontend/contrib/link/cms_plugins.py	85.71%	0 Missing and 1 partial ⚠️
djangocms_frontend/contrib/link/helpers.py	0.00%	1 Missing ⚠️
djangocms_frontend/contrib/link/models.py	0.00%	0 Missing and 1 partial ⚠️
...angocms_frontend/contrib/navigation/cms_plugins.py	83.33%	1 Missing ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #211      +/-   ##
==========================================
- Coverage   86.93%   86.39%   -0.54%     
==========================================
  Files         120      119       -1     
  Lines        3107     3109       +2     
  Branches      330      332       +2     
==========================================
- Hits         2701     2686      -15     
- Misses        304      320      +16     
- Partials      102      103       +1     

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