Fix bugs in handling server names that includes colons

[Twi1ightSparkle]

I ran into this issue when testing something with generic hooks on Hookshot locally where the Synapse server name is localhost:8448

INFO 12:16:27:794 [GenericHookConnection] onGenericHook !QrmxBsBKPajykCohPz:localhost:8448 7e789bf2-7b87-4b10-9438-a22a6f1090d2
ERROR 12:16:27:800 [Appservice] Error registering user: User ID is in use
ERROR 12:16:27:804 [MatrixHttpClient] (REQ-15) {
  errcode: 'M_FORBIDDEN',
  error: 'Application service cannot masquerade as this user (@_webhooks_hookname:localhost).'
}
WARN 12:16:27:804 [Bridge] Failed to handle generic webhook MatrixError: M_FORBIDDEN: Application service cannot masquerade as this user (@_webhooks_hookname:localhost).
    at Object.defaultErrorHandler [as errorHandler] (/usr/bin/matrix-hookshot/node_modules/matrix-bot-sdk/lib/http.js:10:9)
    at doHttpRequest (/usr/bin/matrix-hookshot/node_modules/matrix-bot-sdk/lib/http.js:98:31)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async descriptor.value (/usr/bin/matrix-hookshot/node_modules/matrix-bot-sdk/lib/metrics/decorators.js:19:32)
    at async descriptor.value (/usr/bin/matrix-hookshot/node_modules/matrix-bot-sdk/lib/metrics/decorators.js:19:32)
    at async GenericHookConnection.ensureDisplayname (/usr/bin/matrix-hookshot/Connections/GenericHook.js:271:13)
    at async GenericHookConnection.onGenericHook (/usr/bin/matrix-hookshot/Connections/GenericHook.js:478:13)
    at async /usr/bin/matrix-hookshot/Bridge.js:435:25
    at async Promise.all (index 0)
    at async LocalMQ.<anonymous> (/usr/bin/matrix-hookshot/Bridge.js:410:13) {
  body: {
    errcode: 'M_FORBIDDEN',
    error: 'Application service cannot masquerade as this user (@_webhooks_hookname:localhost).'
  },
  statusCode: 403,
  errcode: 'M_FORBIDDEN',
  error: 'Application service cannot masquerade as this user (@_webhooks_hookname:localhost).',
  retryAfterMs: undefined
}

Signed-off-by: Twilight Sparkle 19155609+Twi1ightSparkle@users.noreply.github.com

[Twi1ightSparkle]

This also solves for if the server name is an IPv6 address

> user = '@foo:1:2:3::4:8448'
'@foo:1:2:3::4:8448'
> user.split(':')
[
  '@foo', '1',
  '2',    '3',
  '',     '4',
  '8448'
]
> user.substring(user.indexOf(':')+1)
'1:2:3::4:8448'

[Twi1ightSparkle]

Searched through the code base for any more instances of .split(':'). Found two in the permissions code, fixed above. AfaIct there are no more instances of this bug