Add a Custom tab

[frangio]

Fixes #110
Fixes #82
Fixes #133

Adds a Custom contract type ("Custom" UI tab and custom contract type for API):

• Pausable, Access Control, and Upgradeability sections.

Adds toggle for Access Control:

• Defaults to false.
• If certain features are selected that require access control, makes Access Control section required (checked and greyed out button) and defaults to ownable.
• Adds API function isAccessControlRequired for every contract type --> used for UI, and can be used by API consumers to determine in what cases would access control be required.

Adds toggle for ERC1155 setURI:

• Adds "Updatable URI" option, true by default (which implies Access Control is enabled by default for ERC1155, but can be disabled by deselecting Updatable URI)

[ericglau]

Ready for review