Do not create empty talk proposals, fix deleting

Ignore empty talk proposals on registration.

Allow deleting talk proposals when editing participants by clearing the title and the abstract.

Improve 'co_m_b_talk_proposal_fields' partial template.
Minor simplification
Autofocus username field in login form
Set the default pagination to 10 per page

Set the default pagination to 10 per page instead of 20.  Taking into account the current interface and the number of participants, browsing participants by paging through is less efficient than filtering of looking up by last name.