refactor(backend): assert token symbol length

[peterpeterparker]

Motivation

In #1550 we will require the assertion of the token symbol length in two functions. That's why I'm extracting here the function to a utility.

Changes

• Move assertion assert_token_symbol_length to a new module assertions.
• Inverse order where the assertion is made - i.e. first assert and then, if everything is ok, parse the eth address.

Tests

The assertion was already covered by an existing test test_add_user_token_symbol_max_length.