[FEATURE REQUEST] Optimize sidebar for single-account

[michaelstingl]

Is your feature request related to a problem? Please describe.
Majority of users have only a single account configured. Those users need to open the account to go to their files.

Describe the solution you'd like
It would be awesome if the single-account could be open by default.

Sidebar
├── 01_Files
├── 02_Shares
│   ├── 01_Shared with me
│   ├── 02_Shared by me
│   └── 03_Shared by link
├── 03_Spaces
│   ├── 01_Space 1
│   └── 02_Space 2
├── 04_Search
│   ├── 01_Saved - All PDF Documents
│   └── 02_Saved - Personal Images
├── 05_Recents
├── 06_Favorites
├── 07_Available Offline
└── 09_Status

Describe alternatives you've considered
n.a.

Additional context
For sharing with ownCloud, we discussed it here:

• [FEATURE REQUEST] Skip account selection when only a one account is configured #1288

TASKS

• Research (if needed)
• Create branch feature/feature_name
• Development tasks
  • Implement whatever
  • ...
• Code review and apply changes requested
• Design test plan
• QA
• Merge branch feature/feature_name into master

PR

• App
• Library (if needed)