feature (standalone gallery): continous check for new pictures

[andi34]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "x" next to an item)

• New feature

What changes did you make? (Give an overview)

From andreknieriem#260

Problem:
When the gallery is used standalone, new pictures won't be visible till the user manually refreshes the page.

Solution:
Added jQuery script that asks the server weather the database changed, if so the page will be reloaded automatically.

Implementation details:

• Updates are detected by checking weather the file-size of the db.txt changed. For this, the db.php got a new function that returns the file-size.
• Gallery.php returns a json response, including the db-file size, if the variable 'status' is set via GET method.
• The regular ajax requests for this are done in a separate js-File (default: every 5s).

Further improvements:

• we need to detect touch and mouse events and only refresh the page if there's no activity

Adjustments by Andreas Blaesius:

• eslint fixes
• adjustments to work on latest Photobooth
• slightly adjust commit message
• make optional available via admin panel
• make refreshing time adjustable

Is there anything you'd like reviewers to focus on?