Support non-equally split shares + various fixes

[tobast]

This is quite a lengthy PR, sorry for the huge review. Fixes #23.

High level overview

• Introduce shares & recurrent shares that are not equally split: a user can specify each consumer's weight for a share. If users u1, u2, u3 participate with respective weights 1, 2, 1.25, for a total of 4.25, u1 would pay 1/4.25th of the price, u2, 2/4.25th of the price, etc. Note that the weights are purely arbitrary and only the fraction user_weight / total_weight counts.
• Introduce all sorts of UI changes to support these without cluttering too much the space.
• Introduce share presets: possibly weighted sets of consumers of a GroupAccount, saved under a name (eg. "rent"). This can come in handy if the same subset of people is often used, and if this group usually requires weighting.
• Introduce a "presets" button in each group panel to add/edit/remove these presets.
• Update to jquery 3.5.1 (1.10.2 was super old); serve jquery locally (do not use a CDN)
• Fix Django-recurrence widget (to select recurrent shares' rules). Seems that this has been broken for a while.

Technical overview

• A non-weighted (equally split) share is just a weighted share with weights of 1.0 for everyone.
• Weighted shares only require adding a custom through model for the ManyToMany relationship between Transactions and UserProfile. This, however, requires a bit of implementation and refactoring.
• The migration that moves from the standard through model to the new custom one is a tricky one — but I tested it heavily and I believe it safe.
• Use path() instead of url() in urls.py (Django 2 style)
• Rename BaseView -> BaseMixin, do not inherit from TemplateView anymore: this was breaking views that were not TemplateViews (eg. CreateView) becasue eg. get() does not generally call the super()-method.