MDL-82344 mod_lti: relax content selection validation in instance form

This was added in MDL-78916 in 4.3 but makes configuring a generic
launch to the default tool URL impossible, since a content item must be
selected. This fix:
- Makes this element optional again, which allows teachers to choose
whether a given instance launches into selected content, or launches
the default tool URL.
- Changes the style to btn-secondary, given it's now optional again