Add custom URL support

[OXeu]

Support https://github.com/szabodanika/microbin/issues/83

What changed

• add MICROBIN_ENABLE_CUSTOM_URL argument, default: false
• replace pasta.id == id to lambda function alias_comparator for better code reuse.
• add custom_alias field in database

Why replace pasta.id == id

There are a large number of statements in the code that compare pasta.id == id, and implementing custom URL inevitably introduces additional fields, requiring a large number of duplicate judgments to be written at all positions. Therefore, I choose to extract the comparison logic into a lambda function to reduce duplicate code.

Todo

Custom URL validity check.