Replaced CircleCI and Travis with GitHub actions

[RiccardoM]

Description

Changes:

• Replaced CircleCI and Travis with GitHub actions (closes #41)
• Added the analyze step that makes sure the code is properly written

Checklist

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

[codecov[bot]]

Codecov Report

Merging #47 into master will increase coverage by 0.12%. The diff coverage is 24.35%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
+ Coverage   18.01%   18.14%   +0.12%     
==========================================
  Files         245      245              
  Lines        5968     5964       -4     
==========================================
+ Hits         1075     1082       +7     
+ Misses       4893     4882      -11     

Impacted Files	Coverage Δ
lib/entities/account/authentication_method.dart	93.33% <ø> (ø)
lib/entities/notifications/notification.dart	78.04% <ø> (ø)
lib/entities/posts/reactions/reaction.dart	62.50% <ø> (ø)
lib/entities/user/user_image.dart	97.50% <ø> (ø)
lib/notifications/notifications_manager.dart	0.00% <0.00%> (ø)
...urces/medias/remote/remote_medias_source_impl.dart	0.00% <0.00%> (ø)
.../notifications/remote/models/fcm_message_data.dart	0.00% <0.00%> (ø)
.../notifications/remote/notifications_converter.dart	0.00% <0.00%> (ø)
...ifications/remote/remote_notifications_source.dart	0.00% <ø> (ø)
lib/sources/posts/remote/helpers/gql_helper.dart	34.37% <0.00%> (ø)
... and 40 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update ef7e1cf...2ecbe64. Read the comment docs.