refactor: change tab title case, and rename schemas to models

[kkopanidis]

refactor: change tab menus to PascalCase
refactor(database): change all mentions of schema to model for clarity
fix(database): skip/limit params in query requests being sent as body
fix(database): custom endpoint documentation pointed to the wrong url

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
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Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx, where "xxx" is the issue number)

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[kon14]

I'd rephrase this a bit, but more specifically is the capital M intended?

ex:
Custom content types are expressed through models, represented as database schemas. Models contain fields of different types, some of which are mandatory while others are optional. Some of them may even accept a default value if none is explicitly provided.