feature(site_notifications): option to mass delete site notifications …

When a user has a lot of site notifications he/she has to delete them
individually. This is very time consuming, so this solves that problem

fixes #6219
docs(actions): Migrated actions section from old Getting Started docs and cleaned up related sections.

Refs #6812.
docs(events): Updated event list and cleaned up existing event docs.

Refs #6812.
docs(hook): Updated hooks docs.

Refs #6812
docs(all): Added getting started for developers.

Refs #6812
docs(all): Cleanup docs.

Refs #6812.
docs(admin): Migrated Getting Started guide from wiki.

Refs #6812.
docs(faqs): Started migrating some FAQs.
chore(users): move river creation to ElggUser::addFriend

Refs #7247
chore(docs): minor change to test the PHP syntax highlighting

Refs #6700
Merge pull request #7249 from juho-jaakkola/highlight_test

chore(docs): minor change to test the PHP syntax highlighting
chore(views): improve static analysis in river views
chore(js): simplifies elgg.ui.initDatePicker logic
docs(js): Adds docs for more JS functions, improves docs for elgg.echo
Merge pull request #7257 from mrclay/js_docs

docs(js): Adds docs for more JS functions, improves docs for elgg.echo
Merge pull request #7255 from mrclay/initDatePicker_logic

chore(js): simplifies elgg.ui.initDatePicker logic
Merge pull request #7254 from mrclay/river_static_analysis

chore(views): improve static analysis in river views
chore(i18n): update translations
chore(release): v1.9.3
Merge pull request #7268 from juho-jaakkola/release-1.9.3

Release 1.9.3