feat: Rename link plugin to text link plugin

[fsbraun]

This PR renames the LinkPlugin to TextLinkPlugin.

• Existing plugins are migrated
• Now, both djangocms-link and djangocms-frontend's link can be installed side by side

Helps solving #218

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.58%. Comparing base (f11feee) to head (1fe1457).
Report is 41 commits behind head on master.

Files with missing lines	Patch %	Lines
djangocms_frontend/contrib/link/cms_plugins.py	66.66%	0 Missing and 1 partial ⚠️
djangocms_frontend/contrib/link/forms.py	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #219      +/-   ##
==========================================
- Coverage   86.93%   86.58%   -0.35%     
==========================================
  Files         120      120              
  Lines        3107     3116       +9     
  Branches      330      333       +3     
==========================================
- Hits         2701     2698       -3     
- Misses        304      316      +12     
  Partials      102      102              

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