Address book: validate chain id; fix download button

[katspaugh]

What it solves

Resolves #2464.

How this PR fixes it

Checks if chain id exists in the list of supported chains.

I've also fixed the Download button in the export modal. The clickable area wasn't covering the whole button.

How to test it

Export the AB on the Arbitrum branch, try to import on this PR's branch. It will not recognize Arbitrum because it's not merged yet.

Screenshots

[github-actions]

CLA Assistant Lite All Contributors have signed the CLA.

[github-actions]

ESLint Summary View Full Report

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Type	Occurrences	Fixable
Errors	0	0
Warnings	0	0
Ignored	0	N/A

• Result: ✅ success
• Annotations: 0 total

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^{Report generated by eslint-plus-action}

[github-actions]

• Safe Multisig app bsc

[github-actions]

• Safe Multisig app rinkeby

[github-actions]

• Safe Multisig app mainnet

[github-actions]

• Safe Multisig app polygon

[francovenica]

Issue:
If there there is a blank space in front of the chainID number the system will take that as a not valid chainID (for example [ADDRESS],[NAME], 56). Not a big issue but pretty annoying

Currently in prod we trim the blank spaces for the chainID so a ", 4" is a valid chainID in prod.

[katspaugh]

@francovenica fixed ✅

[francovenica]

Looks good to me