Design signed data blob for translations

[tsusanka]

During discussions on #987 @prusnak came up with an idea to use one whole memory block in Trezor's storage for signed blob of data received from the Host. The blob will contain mainly the translation strings for the chosen language, however its usage might be broader (e.g. coininfo etc.). We currently assume it would have 128KB.

This blob can be replaced only as a whole. We might split the blob into parts, provide hashes for each part and update only the parts that has changed. However, since uploading 128KB should take about 1s (fair guesstimate) this should be quite seamless for the user. We will consider this optimization later then, if necessary.

This issue covers:

• Design and document how the blob data will be structured.
• Provide API to access the data.
• Research whether this is applicable for T1 as well.
• Research what the is the appropriate blob size. Is 128K the correct size?

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Part of #987. Related: signing the blob #1016, exchange mechanism #1017

[prusnak]

I came up with the following blob structure: https://github.com/trezor/trezor-firmware/blob/master/docs/storage/addon-block.md

Let's discuss this further whether this meets our needs.

[prusnak]

Push the design of the i18n-block + modtrezori18n prototype to thei18n-block branch.

I think the size 128K is fine and this is applicable for T1 as well.

Lowering priority as all of the core tasks are done.

We can continue the work on this once #991 is done.