Fix some spelling/grammar/wording problems in What's New in Grafana v4.0

ref #7469

Update README.md

Adds the Delete function to the Session wrapper so that the Macaron
function for deleting keys from a Session can be used.

https://go-macaron.com/docs/middlewares/session#implement-provider-interface

Add common type for oauth authorization errors

In systemd `After=` merely codes an ordering relation and does not start
other units.

Grafana needs to specify that it wants to start up _after_ either of the
supported DB services have started, _if they start_.

Enabling the database services, however, is out of scope for Grafana.

We now obtain credentials based on the container task's role
rather than just relying on the credentials of the enclosing
container instance.

Fixes #6700.

If the auth config variable, disable_login_form, is set to true then
the username and email fields are set to read-only on the profile page.

The reason for this is so that the user does not lock themselves out by
changing their email address or username. Or create a new user by
changing both.

ref #7810

Since upgrading to Angular 1.6.1, selects with a default empty option
no longer show a popup with an error on submit. This fix adds an
empty option that is then removed with ng-if. Having the default
option set to null or undefined seems to be the cause of the bug.

The list of changes in 1.6 is here but could not find any obvious
cause for this bug:

https://docs.angularjs.org/guide/migration#migrating-from-1-5-to-1-6

Since upgrading to Angular 1.6.1, selects with a default empty option
no longer show a popup with an error on submit. This fix adds an
empty option that is then removed with ng-if. Having the default
option set to null or undefined seems to be the cause of the bug.

The list of changes in 1.6 is here but could not find any obvious
cause for this bug:

https://docs.angularjs.org/guide/migration#migrating-from-1-5-to-1-6

Validate query to avoid self-reference of variable #7614