Unify placement and styling of form buttons.

[linuspahl]

Description

Motivation and Context

With this PR we are starting to unify the styling and placement of our form buttons. Currently their styling, placement and naming varies.

With these changes we are creating a FormSubmit and a ModalSubmit component. The FormSubmit component should be used for forms on pages, while the ModalSubmit should be used for modals which contain a submit button.

By using these components in every suitable form and modal, we are able to change the form and modal submit and cancel button in one place.

With this PR we are also:

• defining more meaningful names for the submit buttons. Instead of Save or Submit we are using for example Create stream
• unifying the naming of the cancel button
• unifying the placement of submit buttons in forms on pages:

Next steps:

• Implementing the FormSubmit and ModalSubmit components for more forms and modals.
• Describing the best practices in our UI documentation

/jenkins-pr-deps https://github.com/Graylog2/graylog-plugin-enterprise/pull/4089