Notifications refactor

[ptbrowne]

To prepare the way for stack managed email layouts, we separate email templating
in two phases.

1. Rendering each individual generic part (emailSubtitle, emailTitle) from Handlebars to MJML
2. Rendering the whole email Handlebars -> MJML -> HTML

To facilitate this process, templating management was centralized inside the Notification
class. Notification children classes now do not do the templating themselves, they only
prepare the data, define a template, and possibily add helpers to Handlebars.

[todo]

Do not pass templateData to global template function

cozy-banks/src/ducks/notifications/Notification.js

Lines 123 to 128 in b5e848d

	// TODO Do not pass templateData to global template function
	// templateData is passed to the global template function for now but it should change
	// at some point since it is the stack that will do this part and it will
	// not receive the application specific templateData
	// To prevent this it would be good to also extract the *parts* inside bank-layout and
	// compile them with the templateData.

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This comment was generated by todo based on a TODO comment in b5e848d in #1577. cc @cozy.