[INF] Unify modifiers

[odyslam]

Description

The codebase makes extensive use of modifiers. There is room for consolidation and simplification to improve the readability of the codebase.

Suggestion

• Remove modifiers from getter functions. The clients that pull the data should be informed when the data returned by these getter functions makes or doesn't make sense. Functions: minimumBid(), lastSettlementTime, holderSolvent, price(), foreclosureTime, cooldownRemaining
• Remove modifiers that offer UX guardrails but no real protocol value: hasFunds
• Remove onlyHolderHeld if onlyHolder AND external are in the same function: setPrice, exit, flagResponse. If it's external, it can't be called by the contract itself. #30

[lekevicius]

Can we separate this issue? I fully agree with removing modifiers from getters and UX guardrails, but less convinced on removing onlyHolderHeld. Maybe that can be moved to a separate issue, and then we can confirm this smaller one?