Add input relay base contract

[guidanoli]

Proposed changes

1. Create a base contract[^1] for portals and relays in contracts/inputs called InputRelay
2. Make portals and relays[^1] inherit InputRelay
3. Remove old base contracts[^1] Portal and Relay

Related issues

Closes #131.

[^1]: and associated interface(s)

[guidanoli]

@ZzzzHui After some thought, I think I agree with you on this. I will revert the moving of contracts. :-)

[guidanoli]

Rebased and reverted moving of contracts. :-)

[guidanoli]

Fixed some paths. :-)

[ZzzzHui]

👍 Just need to rebase the branch

[guidanoli]

Rebased :raised_hands:

[ZzzzHui]

This PR is ready to merge

[guidanoli]

Rebased, and ran yarn prepack.

[guidanoli]

Rebased.