Display correct unit in Qt wallet transaction history

[sphericale]

The Qt wallet transaction history uses 'RVN' for all RVN transactions even if a different display unit is chosen in the options. This has occasionally been a cause of confusion for some users.

These changes:

• Use the correct unit (e.g. RVN, mRVN or μRVN) in the transaction history 'Asset' column and recent transactions
• Removes display unit from the 'Amount' column (as asset amounts are not in RVN)
• Further reduces potential for confusion by displaying asset names in a different colour (I chose raven blue)

[blondfrogs]

utACK - Changes seem fine

[spyder46n2]

Looks like the AWS ARM build failed to compile the external dependencies. This us a semi-random failure and this PR does not appear to be the cause of the build failure. I restarted and will merge once the builds finish.