Discourage fee sniping with nLockTime
Qt: new icons

- flat and b/w icons seems to be the new trend
- support for HiDPI/retina
- change icon sizes
- fix assets-attribution.md
- removed unused icons
Check against MANDATORY flags prior to accepting to mempool

Previously transactions were only tested again the
STANDARD_SCRIPT_VERIFY_FLAGS prior to mempool acceptance, so any bugs in
those flags that allowed actually-invalid transactions to pass would
result in allowing invalid transactions into the mempool. Fortunately
there is a second check in CreateNewBlock() that would prevent those
transactions from being mined, resulting in an invalid block, however
this could still be exploited as a DoS attack.
[Qt] the RPC Console should be a QWidget to make window more independent

- fix issue #5254
libbitcoinconsensus: Add pkg-config support
[Qt] fix missing plural form for a string in sendcoinsdialog

- this was reported on Transifex by pryds (Thank you!)
[Qt] change some strings to reflect name change to Bitcoin Core
[Qt] fix a translation that occurs 2 times but was a little different
Test unexecuted OP_CODESEPARATOR

OP_CODESEPARATOR is an actual executed instruction, not a declarative
thing, so if it's wrapped in an OP_IF it can be turned off.

Using this to implement Rivest's Paywords is left as an exercise for the
reader.
Revised translation process doc
Make IsSuperMajority a standalone function
Move remaining CBlockIndex methods to chain.cpp
Add /rest/headers
Fix download link for gitian builds
Merge pull request #5357

5ea3bc0 Move remaining CBlockIndex methods to chain.cpp (Pieter Wuille)
9dcd524 Make IsSuperMajority a standalone function (Pieter Wuille)
Merge pull request #5471

164a45f Fix download link for gitian builds (BtcDrak)
Apply AreSane() checks to the fees from the network.

'Sane' was already defined by this code as:
 fee.GetFeePerK() > minRelayFee.GetFeePerK() * 10000
 But sanity was only enforced for data loaded from disk.

Note that this is a pretty expansive definition of 'sane': A 10 BTC
 fee is still passes the test if its on a 100kb transaction.

This prevents a single insane fee on the network from making us reject
 our stored fee data at start.  We still may reject valid saved fee
 state if minRelayFee is changed between executions.

This also reduces the risk and limits the damage from a cascading
 failure where one party pays a bunch of insane fees which cases
 others to pay insane fees.
minor fix on script test data docs
Merge pull request #5482

87e8e1c minor fix on script test data docs (Manuel Araoz)
Remove references to X11 licence