[Enhancement] Allow multiple messages to be selected inside conversation view.

[RokeJulianLockhart]

Prerequisites

1. I have searched open and closed issues for duplicates

2. This isn't a feature request

3. This is not a report about my app not working as expected

Problem

Although I am able to perform actions on specific conversations (rather than all or solely one):

...I cannot perform bulk actions to solely specific messages inside conversation view. Instead, I must perform them on all, or solely one at a time:

Solution

Provide checkboxes alongside messages in the conversation view.

Tasks

Status	Task	Solution
Complete	Ask about this at Reddit, to confirm that it is accurate.	I have asked at reddit.com/r/ProtonMail/comments/1nig7l3.
Incomplete	File this as a suggestion at protonmail.uservoice.com/#suggestion_title.
Incomplete	File at ProtonMail/protoncore_android/issues.
Incomplete	File at ProtonMail/ios-mail/issues.

I am waiting for protonmail.uservoice.com/forums/945460-general-ideas/suggestions/50436528 to be implemented, so that this can be filed as a bug via the correct avenue.