TinyMCE: Give Switch Views links more meaningful labels

Fixes CNVS-9799

Test plan:
- Visit every RCE editor in Canvas that offers a control for switching
  between the visual and HTML editor (nearly all of them).
- Verify that the control label is clear about what it does. I.E. not simply
  "Switch Views".
- Switch back and forth between views, and verify that the control always
  works, and that the label is correctly updated when the view changes.

Change-Id: Idc62f57b4914b170428c9e97b7286af177dac269
Reviewed-on: https://gerrit.instructure.com/27989
QA-Review: Myller de Araujo <myller@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>