Add safe send_keys methods to FunctionalTest. Stop implicit wait.

[rmol]

Status

Ready for review

Description of Changes

Add methods to more reliably send key events to elements. This seems
to have fixed one of the most pernicious flaky tests,
test_journalist_interface_ui_with_modal, allowing removal of the
hideous retry hack that could make the tests take minutes longer.

Use them to replace other usages of send_keys, including some that
have definitely been flaky, though not as bad as that one.

Also simplify the safe_click* methods, as WebDriverWait.until returns
the result of the wait condition, and some of the other element
manipulations were unnecessary, given the expected conditions.

Stop setting a default implicit wait in the web drivers. If an element
might not be present, we should use explicit waits instead of adding
this potential delay to every element-locating call. There's also
ample Selenium folklore that mixing implicit and explicit waits leads
to unpredictable timeouts and possible bad behavior.

Rename _alert_wait and _alert_accept.

Double the timeout in alert_wait. This helped with some timeouts
waiting for confirmation popups.

Fixes #4541.

Testing

Run make -C securedrop test. Or just wait for the CI results here.

Deployment

This only involves the functional tests.

Checklist

If you made changes to the server application code:

• Linting (make lint) and tests (make -C securedrop test) pass in the development container

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR

[zenmonkeykstop]

Functional tests are passing locally for me with no issues apart from existing infrequent HOTP and TOTP flakes.
LGTM!