Mattermost-server: Improve display of file and image thumbnails on mobile

[esethna]

See Design and assets: https://projects.invisionapp.com/share/9XRRHMSBV6D#/screens/360642221

Note: If interested in taking on this project please see the supplementary notes on each design slide that detail the functional behaviour. Please also join our Native Mobile Apps channel https://community.mattermost.com/core/channels/native-mobile-apps to ask any questions and receive guidance from our core team

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If you're interested please comment here and come join our "Contributors" community channel on our daily build server, where you can discuss questions with community members and the Mattermost core team. For technical advice or questions, please join our "Developers" community channel.

New contributors please see our Developer's Guide.

JIRA: https://mattermost.atlassian.net/browse/MM-10813

[chuttam]

Mind if I have a go at this one?

[enahum]

@chuttam all yours ;)

[esethna]

@chuttam sweet! Please let us know in the ~Native Mobile Apps channel if you have any questions on implementation or design and we'll be happy to help right away. https://community-release.mattermost.com/core/channels/native-mobile-apps