Create option to hide command log header

**Is your feature request related to a problem? Please describe.**
I'd like to keep the command log visible in the panes but I don't need to see the header text all the time as I know how to hide/focus the panel.

```
┌─Command Log──────────────────────────────────
│You can hide/focus this panel by pressing '@'
│
```

**Describe the solution you'd like**
It'd be nice to have a config option to hide the header text.

Proposed option name:

```yml
gui:
showCommandLogHeader: false
```

(Somewhere [here](https://github.com/jesseduffield/lazygit/blob/cd2c01d1cfb0236ccb7c94a8aae21726ef1283d1/pkg/gui/layout.go#L15) needs a config conditional.)

**Describe alternatives you've considered**

`gui.showCommandLog: false` hides the whole command log which I don't prefer. There needs to be a new option just to remove that sticky header.