add OpenSSL RAND_cleanup() on OpenSSL shutdown

- to securely erase the memory used by the PNRG
- also rework 2 comments in OpenSSL init
[Qt] Replace status bar unit icon with actual images
Add translation mailing list to README.md

Link to the new translation mailing list from README.md
Merge pull request #4401

cf04d83 add OpenSSL RAND_cleanup() on OpenSSL shutdown (Philip Kaufmann)
Merge pull request #4466

ad87bc4 [Qt] Replace status bar unit icon with actual images (Cozz Lovan)
Merge pull request #4477

18c7ba8 Add translation mailing list to README.md (Michael Ford)
replace ComputeMinWork with CheckMinWork
Remove redundant pblock->nNonce = 0
Move UpdateTime to pow
Move CBlockIndex::GetBlockWork() to pow::GetLengthIncrement(nBits)
Simplify and rename CheckWork to ProcessBlockFound
Encapsulate proof of work generation in pow
Encapsulate CBlockHeader fields for pow checks
Encapsulate CBlockHeader fields for pow updates
Proof abstract class
Proof: Cast to ProofOfWork for outputs
Proof::ToString()
FIXME serialize
FIXME tests