fix: review of mining boxes

Description

Adding few missing pieces to the Mining. It is a follow up to the #192 (split into two PRs so it's easier to review)

[x] changed the session structure in Redux from array to the single object (I suppose that we won't keep the history of sessions there)
[x] added missing semi-state in mining boxes (paused but with no sessions)
[x] fixed showing loader to zero values in paused state
[x] added autofocus to inputs in mining boxes
[x] fixed: MiningBox by default doesn't drop any shadow. The shadow should be added only for Tari Mining box when user has to set up wallet.
[x] added missing tags in Merged mining (Ready to set etc.)
[x] fixed wrong icons assignment in merged mining box
[x] added missing case for Header tips above mining boxes

Fixing issues found after another review of the Mining