fix: enabled tokens logic fixes and clarifications

[lekhovitsky]

In this PR:

• CreditManagerV3.maxEnabledTokens is now immutable
• Credit account's enabledTokensMask is set to UNDERLYING_TOKEN_MASK upon opening
• The fact that liquidator can seize non-enabled tokens is disclosed more vocally

Addressed findings:

• https://github.com/spearbit-audits/review-gearbox/issues/70
• https://github.com/spearbit-audits/review-gearbox/issues/79
• https://github.com/spearbit-audits/review-gearbox/issues/81
• https://github.com/spearbit-audits/review-gearbox/issues/89
• https://github.com/spearbit-audits/review-gearbox/issues/95