Improve button wording on speaker management page

"Cancel" implied that the button would revert changes you'd made to add
speakers.

Switch to LCA2020

Remove LCA2018 and LCA2019 styles and content.
Add base LCA2020 styling and update references to year.

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