frontend: rework chroots fields in build forms
Fix #1350
Previously we generated our chroot checkboxes as separate fields within the
form. Each of them having a different name, e.g.
<input name="fedora-32-x86_64" type="checkbox" value="y">
<input name="fedora-33-i386" type="checkbox" value="y">
<input name="fedora-33-x86_64" type="checkbox" value="y">
This is a recommended way of generating checkboxes in many Stack Overflow
threads but it's not flexible enough. Not even getting all ticked checkboxes
isn't easy because how to determine which form fields belong to the checkbox
group ... it needs to be done the other way around by comparing if database
values are actually in the form. This could probably be worked around by
prefixing the field name but this whole thing is just ...
The `MultiCheckboxField` solves the checkboxes generation perfectly. As a
coincidence, it fixed #1350.
I am also throwing away `chroots_sets` from build forms because I don't
see any difference with or without them.
Lastly, I am fixing some tests because since #1350 is fixed, we are now getting
errors that the fedora-18-x86_64 chroot that we want to build in, is not enabled
in the project.