Feature/rich link preview 79

[ryuash]

Description

closes #79 defaults to a placeholder image if not found in meta

EDIT: update preview only displays if we are able to get title/description/image.

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.
• [ ] Updated the documentation.
• [x] Added an entry to the CHANGELOG.md file.
• [x] Re-reviewed Files changed in the Github PR explorer.

[codecov[bot]]

Codecov Report

Merging #97 into master will decrease coverage by 0.25%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
- Coverage   17.76%   17.50%   -0.26%     
==========================================
  Files         258      261       +3     
  Lines        6351     6443      +92     
==========================================
  Hits         1128     1128              
- Misses       5223     5315      +92     

Impacted Files	Coverage Δ
lib/ui/models/link_preview.dart	0.00% <0.00%> (ø)
lib/ui/widgets/post_content/index.dart	0.00% <0.00%> (ø)
...dgets/post_content/widgets/link_preview/index.dart	0.00% <0.00%> (ø)
...ent/widgets/link_preview/utils/preview_helper.dart	0.00% <0.00%> (ø)

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