This commit has the backend code needed to support alerts. From the
frontend perspective, Alerts is separate from pulses and has a
different endpoint, however much of the backend code and database
tables are shared. A few new columns were added to the pulse table to
support alerts, but most of the data is the same between pulses and
alerts.

This also adds several "meta-notifications" around alerts to let users
know when they've been added to an alert, removed from an alert or if
a question has changed such that the alert is no longer meaningful.

This fixes #1372, fixes #6261 and fixes #5768.

This commit has the UI for users to add alerts to a question. Admins
are able to setup alerts for questions and modify alerts that users
have setup.

Handle default values for Dimension (Field Filter) params

Xray insights batch 1

Toggle Shown/Hidden Table Column Settings

Improve performance of toggle all columns in table settings. Add null state to tables with no columns.

Output sampling of X-ray dataset outputs

Use actual URL of the Metastore

update gitignore to include .DS_Store, bin/node_modules, and yarn-err…

updated developer docs frontend unit testing section to use correct y…

* MetaStore integration

* Fix lint error

* restructure embedding levels

* Rename updateSetting in setting widgets to onChange, add onChangeSetting

* embed flow images / spacing

* lint fix

Restyle email templates + pw reset and new user invite emails