Add support for custom PIN length

**Is your feature request related to a problem? Please describe.**
Inside the app, a pin is able to be set that will in turn have to be used to unlock the app. The PIN length is currently fixed at 4 numbers, no more, no less.
This is undesirable as users may want a different length for a plethora of reasons, a key one that comes to mind being that a user might want the PIN to be the same as the one they use to unlock their iPhone, which is usually 6 numbers long.

**Describe the solution you'd like**
Make it to where the PIN length can be customized to any length desired.

**Describe alternatives you've considered**
In this use case, setting a PIN for the app could be considered useless considering it's the same as the unlock password. Some users may find it beneficial to have multiple layers of security though, just in case they were to say accidentally have their phone unlocked, in which case the password would need to be re-entered to get access to, in this case, a user's messages in Element.

**Additional context**
Most of this stuff was said as applying to a hypothetical user. Most of it does apply for me though.