v2.0.0-beta.6

Changes:

• [protonmail] [local store]: Fix Cannot read property 'Labels' of undefined error caused by the recent Protonmail WebClient change, closes #69.
• [local store]: Display messages list in conversation mode by default.

Previous beta releases: v2.0.0-beta.1, v2.0.0-beta.2, v2.0.0-beta.3, v2.0.0-beta.4, v2.0.0-beta.5.