Make sure the gallery doesn't have a navis-before child on open

[joshdarby]

If it does have that child, the bad settings won't be applied.

Changes

This pull request makes the following changes:

• Added another conditional on the navis image click trigger to first check if the gallery has a .navis-before child before firing functions to modify styles and add the X button.

Why

For #1700

Testing/Questions

Features that this PR affects:

• Navis slideshow images

Questions that need to be answered before merging:

• Does it work as expected now?

Steps to test this PR:

Given a single image in an image block that is aligned left or right, take the following steps to reproduce this bug:

1. Click/tap on the image to open the image in a Navis modal.
   1. the modal opens
   2. The image width is set to 100%: https://github.com/INN/largo/blob/e2ac830239452658c285860b330d521c6a0f300e/lib/navis-slideshows/js/navis-slideshows.js#L271
2. Click/tap on the image in the Navis modal.
   1. Nothing happens that the user can see, but the function addCloseX is called again, adding a duplicate X in the modal (which is invisible to the user because it's stacked atop the current X, using the same CSS rules for positioning)
   2. The 100% image width is saved as the pre-click image attribute https://github.com/INN/largo/blob/e2ac830239452658c285860b330d521c6a0f300e/lib/navis-slideshows/js/navis-slideshows.js#L263-L268 and is now relayed to the event handler that calls closeSingle https://github.com/INN/largo/blob/e2ac830239452658c285860b330d521c6a0f300e/lib/navis-slideshows/js/navis-slideshows.js#L274-L276
3. Click on the X to exit the modal.
   1. The 100% from the second click is used as the pre-click image width by closeSingle when the image is closed: https://github.com/INN/largo/blob/e2ac830239452658c285860b330d521c6a0f300e/lib/navis-slideshows/js/navis-slideshows.js#L177
4. The image should still be its normal size. Previously it would be 100% of the column width rather than its previous appearance.

[benlk]

This works.