Improve URL validation and centralise client and server validators

[kylerwsm]

Problem

This PR is a follow-up to #149. Previously, the check for https was disabled in development, as localstack endpoints are not valid urls.

And as suggested by @liangyuanruo, I have centralised the common validators used by our client and server sides.

Solution

• Shift from the built-in Sequelize isUrl check to our customised checks using the validator library. After which, I whitelist the endpoint used by localstack (http://localhost:4572).
• Added a new optional parameter to isValidUrl and isHttps, to indicate whether to take into account whitelisted hostnames when running the validation.
• Created tests for all url validators.

Improvements:

• Validation methods used by the client and server are now centralised at src/shared/util/validation.ts.
• Migrate the above-mentioned validation file to TypeScript.

[liangyuanruo]

does this mean someone will be able to save the whitelisted localhost URL in production?

[kylerwsm]

Technically, users will not be able to using as we are enforcing https during link creation and edits. But I get the gist here. I will convert the whitelist to a dev-only whitelist. Subsequently, if there is a need for a global whitelist in the future, it should be quite easy to add in.

[liangyuanruo]

looks like a good first step towards standardising URL validation. can we guarantee there is no change in behavior?

one possible improvement in the future would be to replace the useWhitelist flag with dependency injection, because the setting is specific to the application environment, and the onus is on the caller to know whether to use a whitelist or not.

[kylerwsm]

I have updated my code such that whitelist is only used when DEV_ENV is true. I was considering the use of dependency injection, which I have decided to drop for now, as it somewhat decentralises the validators again (as the frontend does not have access to environment variables, and have no dependency injection).

[liangyuanruo]

lgtm