Detect if gDM support has been enabled and fallback to the previous gUM
flow otherwise.

Don't show the misleading "user cancelled" error, there are corner cases
in which a different error can be manifested.

- Use getDisplayMedia on browsers where it is supported.
- desktopSharingChromeDisabled and desktopSharingFirefoxDisabled will no longer be supported.

* Switches camera id to mandatory when using old gum flow.

When it fails we retry with different resolutions, and if that doesn't work we remove device id and let gum to decide which device to use.

* Fixes comments.

This is a two part fix:
- In the new gum flow, this.getUserMedia supports promises
  and rejections were not getting caught. So change
  _newGetUserMediaWithConstraints to be promise based instead
  of callback.
- The error name returned from blocked device permissions is
  NotAllowedError, as adapter shims PermissionDeniedError
  to be NotAllowedError.

ESLint 4.8.0 discovers a lot of error related to formatting. While I
tried to fix as many of them as possible, a portion of them actually go
against our coding style. In such a case, I've disabled the indent rule
which effectively leaves it as it was before ESLint 4.8.0.

Instead of failing with an error if another operation is in progress, which is
impossible to know as a user, chain them using the primises API.

CHROME_EXTENSION_USER_GESTURE_REQUIRED will be triggered if
the extension installation fails, because the request to get desktop
stream did not originate in user gesture handler (a button click for
example).

Firefox uses "constraint" as the key in the error:

```
[modules/RTC/RTCUtils.js] <value/<>:  Failed to get access to local media. Error
MediaStreamError { name: "OverconstrainedError", message: "Constraints could be
not satisfied.", constraint: "width", stack: "" } Object { audio: false, video:
Object }  lib-jitsi-meet.min.js:6:10419
```

#416

When calling getUserMedia in chrome for just video with no video devices connected we receive error with name DevicesNotFoundError.