Add validation for tags of length greater than 100 characters

[dojutsu-user]

Fixes #4935

[codecov]

Codecov Report

Merging #4967 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4967      +/-   ##
==========================================
+ Coverage    76.8%   76.82%   +0.01%     
==========================================
  Files         158      158              
  Lines        9937     9944       +7     
  Branches     1241     1244       +3     
==========================================
+ Hits         7632     7639       +7     
  Misses       1973     1973              
  Partials      332      332

Impacted Files	Coverage Δ
readthedocs/projects/forms.py	80.05% <100%> (+0.38%)	⬆️

[codecov]

Codecov Report

Merging #4967 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4967      +/-   ##
==========================================
- Coverage    76.8%   76.77%   -0.03%     
==========================================
  Files         158      158              
  Lines        9937     9959      +22     
  Branches     1241     1247       +6     
==========================================
+ Hits         7632     7646      +14     
- Misses       1973     1980       +7     
- Partials      332      333       +1

Impacted Files	Coverage Δ
readthedocs/projects/forms.py	80% <100%> (+0.33%)	⬆️
readthedocs/core/signals.py	82% <0%> (-4.67%)	⬇️
readthedocs/doc_builder/environments.py	86.27% <0%> (-0.79%)	⬇️
readthedocs/vcs_support/backends/git.py	81.98% <0%> (-0.3%)	⬇️
readthedocs/projects/models.py	85.55% <0%> (ø)	⬆️

[humitos]

Thanks!