Improve slot editing

Fix placement of form to allow submit button to work again.
Display information about slot on the edit popup.

Fix schedule edit

Use main base template instead of intermediate base.
Remove references to Bootstrap 3 and update to Bootstrap 4.
Add edit button to standard grid/mobile view instead of duplicating.

Bootstrapification

Boot custom CSS, and put some base, standard css in its place.
Shame I did not start with fresh Bootstrap4, but oh well.

Some more templates could be made to make this less messy, which would
be good.