Fix a few warts in the frontend code

See merge request !11

Call an API from tahoe-chk as a demonstration that we can

Closes #13

See merge request !9

Also the comment referenced in the comment no longer exists, so stop
referencing it.

In particular, try to give it a factoring that might be maintainable in
practice.

Also, take plaintext from a text input area instead of hard-coding it.

Also switches to the original zfec library version of tahoe-chk since the
Botan version still won't build for Android.

Depend on android-activity and use its `getFilesDir`

Closes #19

See merge request !10

Now that the botan2 derivation is fixed we don't need this.

Also, skip the tests because they're crashing right now. :/

Co-authored-by: Florian <florian@leastauthority.com>

It doesn't seem like it should be necessary but it helps, a little, I think.

It's not clear to me why passing `buildPackages.python3` as the value for
`python` to the `override` call isn't the way to do this.  If we take that
approach then the configure phase fails because it cannot find "python".

It is an Android of some sort.

So none of the newer-compiler-stdenvs nixpkgs offers can help us here.

On Android, use android-activity to discover the right path.  On Linux, use
xdg-datadirs.

The newer tahoe-chk drops the text-format dependency which pulled in many
other complex libraries as transitive dependencies.

This fixes one build error on Android, at least.

However:

```
Configuring android-activity-0.1...
CallStack (from HasCallStack):
  die', called at libraries/Cabal/Cabal/Distribution/Simple/Configure.hs:950:20 in Cabal-2.4.0.1:Distribution.Simple.Configure
  configureFinalizedPackage, called at libraries/Cabal/Cabal/Distribution/Simple/Configure.hs:460:12 in Cabal-2.4.0.1:Distribution.Simple.Configure
  configure, called at libraries/Cabal/Cabal/Distribution/Simple.hs:596:20 in Cabal-2.4.0.1:Distribution.Simple
  confHook, called at libraries/Cabal/Cabal/Distribution/Simple/UserHooks.hs:67:5 in Cabal-2.4.0.1:Distribution.Simple.UserHooks
  configureAction, called at libraries/Cabal/Cabal/Distribution/Simple.hs:178:19 in Cabal-2.4.0.1:Distribution.Simple
  defaultMainHelper, called at libraries/Cabal/Cabal/Distribution/Simple.hs:115:27 in Cabal-2.4.0.1:Distribution.Simple
  defaultMain, called at Setup.hs:2:8 in main:Main
Setup: Encountered missing dependencies:
base >=4.9 && <4.11
```