Add information about last time manga and chapters were fetched from source

[martinek]

This PR fixes #428. It contains:

• migration to add columns on manga where data will be stored
• changes to add new columns to API
• changes to update the data when manga / chapters are fetched from source
• "virtual" properties on manga to show calculated age of data in API

I am not sure about the way age is calculated, but I could not figure out, better way to do that.

[AriaMoradi]

You could merge the two migrations into one by writing a sql migraion
https://github.com/Suwayomi/exposed-migrations/blob/master/lib/src/main/kotlin/de/neonew/exposed/migrations/helpers/AddColumnMigration.kt

[martinek]

You could merge the two migrations into one by writing a sql migraion https://github.com/Suwayomi/exposed-migrations/blob/master/lib/src/main/kotlin/de/neonew/exposed/migrations/helpers/AddColumnMigration.kt

Is that better then using helper from library though? If there was helper for multiple columns, that would be better, but custom SQL for something that can be done with lib feels less safe to me.

[AriaMoradi]

If there was helper for multiple columns, that would be better

Well, If you are uncomfortable with doing a direct SQL Migration, make a PR to the migration lib, we own it.

[martinek]

I have added the custom sql migration.