0.7.4

(core) lightly freshen the core readme, mentioning roadmap and forums…

… etc.

Summary: This is a documentation update, and version bump on grist-core.

Test Plan: No code changes.

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2982

(core) Fix several flaky tests.

Summary:
- Fix flaky SamplesWS test, which was occasionally clicking a disabled button
- Increase timeout on backupSqliteDatabase test, which sometimes times out
- Fix a little flakiness in ExportSection test.
- Fix flaky Billing test by adjusting behavior slightly.

In Billing, when re-fetching subscription (e.g. on navigating back), we now
unset it while the fetch is pending, so that billing pages show a spinner or
"Fetching..." messages. This also gives tests something to wait for.

Also adjusts Billing styles on the "Fetching..." messages to make them look
different from regular text.

Test Plan: Mainly making exising tests more robust. Billing changes exercised by existing tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2920

(core) add CI github action for grist-core

Summary:
Activate CI for grist-core using github actions.  Can be improved
with caching but starting simple.

Test Plan: tested in a fork of grist-core

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2771