client: replace submodule with specific release

[alxndrsn]

TODO

• remove submodule
  • confirm behaviour when update is pulled which removes the submodule - does the existing dir remain in place, with corresponding .git dir
• support build options
  • test files (via mounting a dist/ directory?)
  • build from source
  • pull from version (default for docker-compose.yml)

---

Closes #1996

What has been done to verify that this works as intended?

• tested locally
• tested in CI
• tested with dev builds

Why is this the best possible solution? Were any other approaches considered?

• 👍 very fast, low overhead container builds
• 👎 doesn't allow local edits
• 👎 doesn't currently allow arbitrary treeish refs

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

• 👍 should help 3rd-party deployers who don't edit server/central-backend code
• 👎 should confuse 3rd-party deployers who do edit server/central-backend code

Does this change require updates to documentation? If so, please file an issue here and include the link below.

Before submitting this PR, please make sure you have:

• branched off and targeted the next branch OR only changed documentation/infrastructure (master is stable and used in production)
• verified that any code or assets from external sources are properly credited in comments or that everything is internally sourced

[alxndrsn]

Delete this line.

[alxndrsn]

Suggested change

FRONTEND_REPO: alxndrsn/odk-central-frontend

[alxndrsn]

Use a real version or RC.