WIP: [TM-274] Update FA1.2

Problem:

1. FA1.2 is not fully usable in current version of Michelson. One can not call methods of an abstract FA1.2-compliant contract from another contract because it requires knowledge of concrete parameter type which is not specified in the document.
2. FA1.2 has a composability issue: if there appears a new standard interface with a similar requirement on its parameter, it will become impossible for a contract to comply with both requirements at the same time.
3. There are some typos in FA1.2 description and some entrypoints are described in subjectively bad way.

Solution:

1. Update "Approvable Ledger Interface" so that it only requires presense of certain entrypoints, but does not require anything regarding their position in the parameter tree.
2. Describe a temporary strict version of this interface which makes usable in Athens.
3. Ensure consistent spelling of "entrypoint(s)", that's how this word is spelled in Michelson docs.
4. Rephrase descriptions of some entrypoints, fix some minor typos and mistakes.
5. Update FAQ.

TODO:

• Update assets (provide implementations for Athens and Babylon). See morley-framework/morley!31 (merged)