Add a helper methods to FlagServiceInterface to determine if an entity is flaggable

Problem/Motivation

There is quite a bit of logic implemented in FlagService::flag() that may through exceptions when flagging an entity:

    // Check the entity type corresponds to the flag type.
    if ($flag->getFlaggableEntityTypeId() != $entity->getEntityTypeId()) {
      throw new \LogicException('The flag does not apply to entities of this type.');
    }

    // Check the bundle is allowed by the flag.
    if (!empty($bundles) && !in_array($entity->bundle(), $bundles)) {
      throw new \LogicException('The flag does not apply to the bundle of the entity.');
    }

    // Check whether there is an existing flagging for the combination of flag,
    // entity, and user.
    if ($this->getFlagging($flag, $entity, $account)) {
      throw new \LogicException('The user has already flagged the entity with the flag.');
    }

And in FlagService::unflag():

    // Check the entity type corresponds to the flag type.
    if ($flag->getFlaggableEntityTypeId() != $entity->getEntityTypeId()) {
      throw new \LogicException('The flag does not apply to entities of this type.');
    }

    // Check the bundle is allowed by the flag.
    if (!empty($bundles) && !in_array($entity->bundle(), $bundles)) {
      throw new \LogicException('The flag does not apply to the bundle of the entity.');
    }

    $flagging = $this->getFlagging($flag, $entity, $account);

    // Check whether there is an existing flagging for the combination of flag,
    // entity, and user.
    if (!$flagging) {
      throw new \LogicException('The entity is not flagged by the user.');
    }

Proposed resolution

Add helper methods, either to FlagServiceInterface to determine if an entity is flaggable and unflaggable.

Remaining tasks

User interface changes

API changes

Data model changes