Add Parachain Template (#620)

* Add Canvas node as Parachain template

* Remove `pallet-contracts`

* Point to local Cumulus dependency

* Use double quotes instead of single quotes

* Get rid of GPL licensing

* Remove references to Canvas

* Get rid of warnings

* Remove GLP-3 License copy-pasta file

* Copy in README from `substrate-parachain-template`

* Add mention of `polkadot-launch` tool

* Add missing screenshot asset

* Remove Canvas hidden files and scripts

* Rename `template` to `parachain-template`

* Remove a few more Canvas references

* Add `pallet-template`

* Get `pallet-template` compiling

* Remove TODOs about Weights

* Sort some dependencies

* Remove contracts specific const

* Change binary name back to `parachain-collator`

* RustFmt

* Fix mock tests

* Purge sneaky whitespace

* Add template pallet index to runtime

Co-authored-by: Ricardo Rius <9488369+riusricardo@users.noreply.github.com>

* Add force authoring to collator `polkadot-launch` config

Co-authored-by: Ricardo Rius <9488369+riusricardo@users.noreply.github.com>

* Refer README readers to `substrate-parachain-template`

* Remove license header in `build.rs`

Co-authored-by: Michael Müller <michi@parity.io>

* Fix punctuation nitpick

Co-authored-by: Michael Müller <michi@parity.io>

* Remove unused `lib.rs` file

* Add note about Rococo network

Co-authored-by: Ricardo Rius <9488369+riusricardo@users.noreply.github.com>
Co-authored-by: Michael Müller <michi@parity.io>