Skins - cannot Save changes once background image file previewed

Patch for "master" branch: https://reviews.mahara.org/10662

Retested - failed

- Browsers tested: Edge, Chrome and Firefox
- Themes: Primary school(Person Bob) and Raw (Person admin)

Steps
Preconditions:

1. A logged in person has at least one image in their home files folder and at least one sub-folder named 'images' with several image files in it.

Test steps: (repeat these for each browser/theme)

1. Person selects to create a new skin (Main menu|Create|Skins--> Create skin - or edit an existing one )
2. Select the 'Page' tab
3. Select the Page background image's 'Add a file' button and select the 'image' sub-folder
4. Select a filename to preview a file's content
5. Close the Download/preview modal
5. Select the tick to select a file ✘
6. Save the page
Actual result: (see Edge attachment) when the user selects to close the download/preview window, the 'Add a file' modal also closes.
Work around: the user can reload the 'Add a file' modal and select an image (WITHOUT previewing it!) by selecting the file's tick then Save it.

Please note that the above happens in all three browsers (I couldn't edit it once saved).

Reviewed: https://reviews.mahara.org/10662
Committed: https://git.mahara.org/mahara/mahara/commit/be433e76fe897b6f645150a424e4e62c4ae2ac81
Submitter: Robert Lyon (<email address hidden>)
Branch: master

commit be433e76fe897b6f645150a424e4e62c4ae2ac81
Author: Lisa Seeto <email address hidden>
Date: Tue Dec 10 11:39:02 2019 +1300

Bug 1848100: Skins - cannot Save changes once background image file previewed

- passed down prefix in filebrower.tpl to pagemodal.tpl to create
unique filebrowser modal id's, modified jq to target divs
- optionally set page-modal id, adjust id's affected
- include missing paginator script
- alter data-dismiss to be conditional, added jq to close button event

behatnotneeded

Change-Id: If4d267ef65fce010fe9d8d242c56dd3bac331476
Signed-off-by: Lisa Seeto <email address hidden>