base: Further changes to base.html template blocks
templates: Adds base_single.html template which wraps the content in panel-success
allauth: Add email.html template for overriding
roots_tags: Add {% define %} template tag

Adds {% define %} template tag that is used for defining a template
variable with multiline content that is added to the template's context.

Within the template tag the content can also contain other template code,
including blocks which can be extended by the templates that are inheriting
from this template.
base: Add panel.html snippet for middle_panel
templates: Leverage panel.html with define blocks to avoid code duplication
base: Add content_wrapper block to the base.html template
templates: Add allauth's templates for overriding
allauth: Change templates to use base_single.html look
photologue: Add margin around photos in the gallery
competitions: Leverage crispy_forms in CompetitonRegistratonForm
base: Remove redundant template files from base directory
events: Make event_list template leverage panel.html
competitions: Change constraints for making a series active

- Remove a constraint that allows only one active series per competition
- Add a constraint that disallows making a series active when it is past
  its due date
problems: Change problem_list.html template tu use base_single.html
problems: Remove ProblemList view from urls and menu
base: Add carousel that displays photo slideshow
competitions: Replace SeasonJoinView with SeasonListView

Rather than displaying a silly form that requires yet another step
before user can see the problems and participate, simply list
all the problems of the active series.
competitions: Add wiring for the SeasonList view
menu: Add link to the SeriesListView from the menu