Update button styles

Replace Bootstrap 3 button default style with Bootstrap 4 equivalent.

Simplify django form rendering

Remove my hack to get bootstrap forms.

Remove the insane render-row-at-a-time forms that then only render
default django forms.

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.

More dashboard and settings themes (#29)

* Proposals forms now have a header panel

* Adds panels to the speaker forms

* Imports latest pinax accounts templates

* speakers/base and proposals/base now derive from utility_page.html

* Most login forms now derive from take flight themes

* Missed email_confirm

* More accounts bits

* Restructures login form

* Puts submit etc buttons inside fieldset borders

Refactors all speaker-facing forms and editing pages to use the new theme (#27)

* Themes speaker_edit.html

* speaker_create now uses takeflight theme

* proposal_submit_kind now uses takeflight theme

* fixes bug in form_snippet

* Optimises proposals_submit_kind

* proposal_submit now uses wagtail theme

* Proposal_edit now uses new theme

* More forms

* Proposal details page now loads from take flight theme

* proposal_fields now looks nicer under the take flight theme

* proposal_speaker_manage rethemed