Port Flex

[cedelavergne-ledger]

Port the app to support Flex.

This includes:

• Port NBGLv2
• Minor improvements in navigation and About menu display
• Deactivation of the decryption with the curve cv25519 (#84)
• Add Flex config
• Update snapshots

Checklist

• [X] App update process has been followed
• [X] Target branch is develop
• [X] Application version has been bumped

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 27.01%. Comparing base (74e65a4) to head (9152222).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #107 +/- ## ======================================== Coverage 27.01% 27.01% ======================================== Files 1 1 Lines 211 211 ======================================== Hits 57 57 Misses 154 154 ``` | [Flag](https://app.codecov.io/gh/LedgerHQ/app-openpgp/pull/107/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LedgerHQ) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/LedgerHQ/app-openpgp/pull/107/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LedgerHQ) | `27.01% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LedgerHQ#carryforward-flags-in-the-pull-request-comment) to find out more.

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