Allow the date- and time-format strings to contain escaped characters

[carstingaxion]

The current conversion from php dateformat into javascript moment format is not able to handle escaped characters.

Description of the Change

My requested change makes the format conversion aware of escaped characters and tells it, to just go on, in such case.

Closes #632

How to test the Change

1. Go to Events > Settings and set the date format as "j \d\e F \d\e Y"
2. Create an event
3. Make sure the shown date in the editor looks like "DAY de MONTH de YEAR"

Changelog Entry

Fixed - Bug fix Event date display in the editor when using escaped characters in date or time format

Credits

Props @carstingaxion

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[carstingaxion]

It's a little bit annoying to wait for php unit tests to finish, when the only changed files were JS files.
We should consider some separation at this point.

For the environment 🌴 🌳 🌲 !

[carstingaxion]

Ok, all tests passed, but the one DB issue comes up again

[carstingaxion]

Ok, all tests passed, but WordPress/plugin-check-action#61 comes up again

... which is a little weird because this branch hasn't even the workflow-file available.