feat: progressive web app support (PWA)

[faustbrian]

Makes it possible to use the paper wallet offline by installing it as a local application through your browser.

PWAs only work over HTTPS @ItsANameToo. Easiest to run it is to locally do valet secure paper-wallet for testing to get a local SSL certificate.

Draft because I'm waiting for images from Oleg.

What kind of change does this PR introduce?

• Bugfix
• Feature
• Refactor
• Performance
• Tests
• Build
• Documentation
• Code style update
• Continuous Integration
• Other, please describe:

Does this PR introduce a breaking change?

• Yes
• No

Does this PR release a new version?

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the develop branch, not the master branch
• All tests are passing
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

[codecov-io]

Codecov Report

Merging #29 into master will increase coverage by 4.42%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
+ Coverage    36.9%   41.33%   +4.42%     
==========================================
  Files           6        5       -1     
  Lines          84       75       -9     
  Branches        3        3              
==========================================
  Hits           31       31              
+ Misses         53       44       -9

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