Cache all messages

**Is your feature request related to a problem? Please describe.**
I have a very limited mobile data plan, but unlimited internet at home. Sometimes when I am away from home I would like to lookup some old messages but scrolling up is really costly data wise. Also, for some reason Element Android seems to forget old messages/does not load them if it is missing some messages in between.

**Describe the solution you'd like**
It would be awesome if all messages one receives would be saved offline so one can always scroll up without using data.

**Describe alternatives you've considered**
An alternative/addition could be that one could download all messages (i.e. backfill the history) via a button.

**Additional context**
This would make the client be more like e.g. WhatsApp, where one can read through ones messages without having to be connected to the internet.
It also seems to be the case that the new search feature will only be able to search cached messages. By implementing this the search feature would be way more useful for encrypted chats (so most 1:1 chats).