add migration for base_url

[kerkness]

It would appear that the baseUrl feature requires a database column that isn't included in the migrations.

[kerkness]

Added an if to the up() method for those who already added the column.

[cesargb]

This functionality is only required when we create the MagicLink. Once it is created, it is not necessary get the url again.

There is a test that verifies its correct operation:

https://github.com/cesargb/laravel-magiclink/blob/46ee75d32a556b02a0d154041b32eca8d1a5737b/tests/MagicLinkTest.php#L133

If you believe that there is any reason why it is necessary to obtain the MagicLink url long after its creation and that it is justified, I see no problem in adding this field, but at the moment I cannot think of any reason.