Toasts and search

[thibaudcolas]

Fixes one issue and implements one feature:

• Removes Bootstrap toast styles, which interfere with the Read the Docs Addons’ "readthedocs-notification" component, which also uses the toast class. We don’t need this part of Bootstrap so might as well remove it.
• Triggers the Read the Docs Addons’ search modal, if RTD Addons are initialised on the page.

Both of those issues are impossible to properly test locally but can be checked with the PR preview build.

[laymonage]

Thank you!