Post URL preview fix

[RiccardoM]

Description

This PR fixes the slowness introduced in the last version due to a bad URL preview implementation. It also fixes a build problem and prepares the project to be built using the latest Flutter 1.20.0 stable version. Fixes #119

Checklist

• [x] Targeted PR against correct branch.
• [x] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• [ ] Wrote unit tests.
• [x] Run flutter format.
• [x] Updated the documentation.
• [ ] Added an entry to the CHANGELOG.md file.
• [x] Re-reviewed Files changed in the Github PR explorer.

[codecov[bot]]

Codecov Report

Merging #118 into master will increase coverage by 1.77%. The diff coverage is 7.51%.

@@            Coverage Diff             @@
##           master     #118      +/-   ##
==========================================
+ Coverage   17.56%   19.33%   +1.77%     
==========================================
  Files         261      261              
  Lines        6456     6573     +117     
==========================================
+ Hits         1134     1271     +137     
+ Misses       5322     5302      -20     

Impacted Files	Coverage Δ
lib/entities/posts/link_preview.dart	0.00% <0.00%> (ø)
...osts/remote/converters/link_preview_converter.dart	0.00% <0.00%> (ø)
lib/ui/blocs/navigator/navigator_event.dart	0.00% <ø> (ø)
lib/ui/blocs/post_details/post_details_bloc.dart	0.00% <0.00%> (ø)
lib/ui/blocs/posts_list/posts_list_bloc.dart	0.00% <0.00%> (ø)
lib/ui/screens/export_mnemonic_screen/index.dart	0.00% <0.00%> (ø)
...een/widgets/home_screen_top_bar_account/index.dart	0.00% <0.00%> (ø)
...creen/widgets/home_screen_top_bar_posts/index.dart	0.00% <0.00%> (ø)
...me_screen/widgets/mnemonic_backup_popup/index.dart	0.00% <0.00%> (ø)
...reen/widgets/notifications_main_content/index.dart	0.00% <0.00%> (ø)
... and 89 more

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