Static Integration DSL: BaseSlashCommands

Convert Integrations::BaseSlashCommands to use the static field DSL introduced.

Proposal

• Update the class with:

field :token,
  type: 'password',
  non_empty_password_title: -> { s_('ProjectService|Enter new token') },
  non_empty_password_help: -> { s_('ProjectService|Leave blank to use your current token.') },
  placeholder: 'XXxxXXxxXXxxXXxxXXxxXXxx'

• Remove prop_accessor :token and def fields.
• Update the relevant specs