User can add multiple instances of a series into a collection

[php-coder]

To be done:

• relax constraints on database level (/collection/{slug}: exception happens when user adds a series to collection that already has this series #986)
• make it possible to add more than one instance of a series
• update a view to show a list of series
• make it possible to delete only one instance of a series from a collection
• protect deletion against removing someone's else instance (we've already protected by the SQL query that we use)
• don't show "New" badge multiple times (I'll deal with it in a separate issue)
• customize "Series isn't part of your collection" message => "Add another one instance of this series to collection"
• integration test
  • the form is visible again
  • the message has been changed
  • a number of stamps is displayed

[php-coder]

It seems like using UUID type isn't good idea:

• it can be slow: http://www.h2database.com/html/functions.html#random_uuid
• it is supported by MySQL only since version 8
• even if we would have the latest version of MySQL, Liquibase seems doesn't know about it: https://github.com/liquibase/liquibase/blob/02ffb85e1028038d8c07821fe89575e70c137526/liquibase-core/src/main/java/liquibase/datatype/core/UUIDType.java#L39