fix: height option for Markdown field

[mihaimdm22]

Same logic as code, if not present or wrong height measure (ex: '3maini') the height of the field will be auto (expand as long as it's content). Unfortunately the MavonEditor for markdown has a minimum height of 300px that cannot be override, so for values less than 300px, the editor will have height 300px, otherwise setting more than 300px will set the height accordingly.

height: 4degetemici => auto height
height: 125px => height: 300px
height: 375px => height: 375px
height: 1cm => height: 300px
height: 10cm => height: 377.49px

[codecov]

Codecov Report

Merging #180 into master will increase coverage by 0.63%.
The diff coverage is 94.29%.

@@            Coverage Diff             @@
##           master     #180      +/-   ##
==========================================
+ Coverage   96.03%   96.66%   +0.63%     
==========================================
  Files          87      132      +45     
  Lines        1890     3628    +1738     
==========================================
+ Hits         1815     3507    +1692     
- Misses         75      121      +46     

Impacted Files	Coverage Δ
lib/avo/app/tools_manager.rb	50.00% <50.00%> (ø)
spec/dummy/app/policies/user_policy.rb	56.25% <53.33%> (ø)
spec/dummy/app/avo/filters/published_filter.rb	66.66% <66.66%> (ø)
lib/avo/app/action.rb	80.00% <80.00%> (ø)
lib/avo/engine.rb	85.71% <85.71%> (ø)
lib/avo/app/fields/boolean_field.rb	93.75% <87.50%> (ø)
lib/avo/app/app.rb	89.70% <89.70%> (ø)
lib/avo/app/fields/key_value_field.rb	90.90% <90.90%> (ø)
.../avo/app/fields/field_extensions/has_field_name.rb	91.66% <91.66%> (ø)
...lds/field_extensions/visible_in_different_views.rb	92.85% <92.85%> (ø)
... and 173 more

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