[BUG] Toolbar back button bad

[APP]
**Problem**
Currently our back button links straight back to the dashboard no matter where it's pressed.

**Where/When**
On any page with a back button, eg `/sessions/:sessionid`

**Desired behavior**
Should go logically out one level. eg clicking back on `/sessions/:sessionid` should go back to `/sessions` and clicking it again should go back to `/`. This is different to just doing `history.go(-1)` as this would cycle back through every page you've visited before getting back to the dashboard.

**Additional context**
The back button lives inside [`Toolbar.vue`](https://github.com/codersforcauses/p2s-v2/blob/d10a79649f73d773634d2123fa7dfecba3a090f7/app/src/components/layout/Toolbar.vue#L3-L9)