Displaying number of messages (for all messages and for specific "channels")

**Is your feature request related to a problem? Please describe.**
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

In most cases you would delete old messages because it was only an information for a task
of a server (e.g. after backups). An information about the number of messages would be
useful for tidy up the messages which are too old and no longer of interest for the user.

**Describe the solution you'd like**
A clear and concise description of what you want to happen.

Displaying the numbers of messages for "All messages" and for each "channel" the user has created.
It should be displayed to the right of the text on the left side of the screen.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.
Counting the messages myself ;-)

**Additional context**
Add any other context or screenshots about the feature request here.
Not necessary.