Fix hostname validation

[kfdm]

• Move validation into forms.HostForm
• Minor refactoring of views.HostRegister

[codecov-io]

Codecov Report

Merging #181 into master will increase coverage by 0.15%.
The diff coverage is 76.19%.

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
+ Coverage   47.11%   47.26%   +0.15%     
==========================================
  Files          43       43              
  Lines        2424     2435      +11     
==========================================
+ Hits         1142     1151       +9     
- Misses       1282     1284       +2

Impacted Files	Coverage Δ
promgen/views.py	50% <62.5%> (-0.15%)	⬇️
promgen/forms.py	93.13% <84.61%> (-1.25%)	⬇️

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