Add features to allow messages to be sent without scrolling to end of chat.

[ianmacd]

This patch adds a settings toggle to globally prevent the conversation window from scrolling after sending a message. A facility to allow the setting to be overridden on a per-message basis is also provided,

Description

When catching up on a busy conversation, it is often convenient to send messages without scrolling the conversation window as one ploughs through the backlog of messages. This enables the user to maintain his reading position in the conversation until he has caught up, thereby also preserving the unread message count for the group.

Most people probably won't want to globally disable scroll-on-send, however, so this patch further allows the user to deactivate scroll-on-send on a per-message basis by submitting the message with [Alt] + [Enter] instead of just [Enter].

Contributor checklist:

• [x] My commits are in nice logical chunks with good commit messages
• [x] My changes are rebased on the latest clearnet branch
• [x] A yarn ready run passes successfully (more about tests here)
• [x] My changes are ready to be shipped to users

[KeeJef]

Happy to merge this, but we should do cross platform and after the settings redesign which we are working on now.

[KeeJef]

How are people feeling about this one? Haven't seen too much popular support since this was raised, would be a little bit of work to do cross platform

[KeeJef]

Closing this PR for now as we haven't received many requests for this feature from the community, support emails, or GitHub. Will will reopen it if we see a larger demand in the future.